The invention relates to measuring stress levels in polymers or polymeric compositions.
Processes for manufacturing polymeric compositions (e.g., adhesives such as structural adhesives) often require adding or combining precise amounts of pre-polymeric components forming these compositions, particularly where these components react together to form the composition. Devices dispensing these components can malfunction periodically and/or systematically, resulting in the deposition of an incorrect mix of the components. These malfunctions can significantly affect the quality of the resulting products.
It is also desirable to be able to measure the quantity of polymeric or pre-polymeric material in any given volume of an article incorporating the material. For example, in the case of a structural adhesive joining two substrates together, it is desirable to measure the thickness of the adhesive throughout the adhesive joint to determine whether the thickness is uniform. Non-uniformities can affect the performance of the joint, causing it to under perform in some circumstances.
It is also desirable to be able to measure the level of stress in a polymer or polymeric composition or substrate.
In general, the invention features a method for measuring the level of stress in polymers or polymeric compositions within a given volume that includes providing a polymeric or pre-polymeric composition containing a plurality of microparticles having a non-ferromagnetic or non-ferrimagnetic core provided with a coating that is ferromagnetic or ferrimagnetic wherein the microparticles are substantially uniformly dispersed throughout the composition. The microparticles have a detectable magnetic characteristic which correlates with the level of stress in the composition.
In another aspect, the invention provides a method for measuring the level of stress in polymers or polymeric compositions within a given volume that includes combining the polymeric or pre-polymeric composition with a plurality of microparticles having a non-ferromagnetic or non-ferrimagnetic core provided with a coating that is ferromagnetic or ferrimagnetic to form an admixture in which the microparticles are substantially uniformly dispersed throughout the composition. The microparticles have a detectable magnetic characteristic which correlates with the level of stress in the composition.
In another aspect, the invention provides a method for determining the stress level in said admixture during dispensing of said admixture.
In another aspect, the invention provides a method for determining the stress level in a polymeric reaction mixture.
In another aspect, the invention provides a method of measuring inductance or inductive reactance of a sample comprising the steps of measuring the inductance or the inductive reactance of the sample; and correcting said measurement of inductance or inductive reactance for temperature.
As used herein, a xe2x80x9cpre-polymeric compositionxe2x80x9d refers both to compositions whose molecular weight has not been sufficiently advanced to qualify as a polymeric composition (e.g., partially polymerized pre-polymeric syrups), as well as individual reactants in the form of monomers or oligomers that react with themselves or with other reactants to form a polymeric composition.
In preferred embodiments, the core of the microparticles is selected from the group consisting of glass bubbles, glass beads, glass fibers, fumed silica particles, fused silica particles, mica flakes, polymeric particles, and combinations thereof, with glass bubbles being particularly preferred. The coating (which may be provided over substantially all or a portion of the surface of the core) is preferably a ferromagnetic or ferrimagnetic material. Examples of suitable ferromagnetic or ferrimagnetic materials include nickel, iron, alloys thereof, and oxides thereof. Stainless steel coatings are particularly preferred.
The microparticles preferably have an average major dimension between about 10 micrometers and about 1 millimeter. The average thickness of the coating preferably ranges from about 0.1 nanometers to about 5 micrometers, more preferably from about 1 nanometer to about 200 nanometers. The amount of microparticles provided in the admixture preferably ranges between about 0.01 and 50% by volume.
In one preferred embodiment, the method of the invention is used to determine the level of externally applied forces on the composition.
In another preferred embodiment, the method of the invention is used to measure the degree of cure of an adhesive composition.
In another embodiment, the method of the invention is used to determine the quality of adhesion of an adhesive or adhesive composition to a substrate.
In another embodiment, the method of invention is used to determine the direction of internal stress of a polymeric composition.
One example of a useful polymeric composition is an adhesive composition. Specific examples of preferred polymeric compositions include epoxy resins (e.g., base-cured epoxies, acid-cured epoxies, and addition-cured epoxies), polyurethanes, acrylates, polyorganosiloxanes, and phenolics.
The invention provides a reliable method for measuring the levels of stress in a polymeric or pre-polymeric composition within a given volume using microparticle xe2x80x9ctagsxe2x80x9d having a detectable magnetic characteristic. The measurement of the stress level in a polymeric composition can then be correlated with and used to determine, for example, the degree of cure of an adhesive or polymer composition, the level of applied external forces, the level or quality of adhesion of an adhesive to a substrate and the internal stress due to heating and cooling cycles. The microparticles are easily fabricated and are generally chemically inert and stable over reasonable periods of time. The microparticles can also be used to determine the volume of the composition as described in copending patent application entitled xe2x80x9cMethod for Measuring the Quantity of a Polymeric or Pre-polymeric Composition,xe2x80x9d U.S. application Ser. No. 08/610,605, filed on Mar. 8, 1996, the entire contents of which is incorporated herein by reference.
Moreover, certain properties of the microparticles are very similar to those of their uncoated counterparts. For example, metal-coated glass microbubbles impart substantially the same Theological behavior and mechanical properties as their uncoated counterparts. Thus, the microparticles can be substituted virtually one-for-one for their uncoated counterparts on a volume basis without adversely affecting the properties of the final composition.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.